This invention relates to phased array antennas, and more particularly to a method for positioning a phased array antenna disposed on a rotatable platform, which is in turn disposed on a moving platform such as an aircraft, such that the antenna can be pointed as needed to track a desired target such as a satellite-based radio frequency transponder.
Electronically scanned phased array antennas typically exhibit superior tracking of a target, such as a satellite, when compared with purely mechanical dish antennas or single-axis electronically scanned phased array antennas. However, phased array antennas generally lose performance rapidly as the elevation angle above the plane of the array drops below about 30xc2x0. This can pose a difficulty in tracking a target such as a satellite-based RF transponder, when the antenna is mounted on a relatively fast moving mobile platform such as a jet aircraft. Depending upon the location of the aircraft relative to the geostationary arc of the satellite-based RF transponder being tracked by the aircraft""s antenna, the elevation angle may drop below about 30xc2x0 at one or more times during travel of the aircraft, thus causing a degradation in antenna performance and, at the worst case, loss of the signal being received from the satellite-based RF transponder.
Accordingly, it would be highly desirable to be able to mechanically move (i.e., rotate) an electronically scanned phased array antenna employed on a moving platform, as needed, to ensure that the antenna is positioned at all times to maintain the elevation angle of the beam of the antenna such that the elevation angle never drops below a predetermined lower limit.
Rotating the antenna whenever needed to maintain the scan angle greater than 30xc2x0 (or other predetermined limit) would obviously require a significant number of positioning adjustments to the antenna when the antenna is used with a relatively fast moving platform such as a jet aircraft, which can cover a large distance in a relatively short time. Rotating the antenna as often as needed to maintain the elevation angle of the antenna beam to within a predetermined range could therefore result in excessive use of the apparatus used to reposition the antenna (i.e., most likely a motor) and premature mechanical failure of the apparatus.
It is therefore a principal object of the present invention to provide an apparatus and method for positioning a planar, electronically scanned, phased array antenna disposed on a moving platform such that the antenna can be rotationally positioned as needed during operation of the moving platform to maintain the elevation angle of the beam of the antenna within a predetermined range. In this manner, antenna performance can be maintained even if the moving platform would otherwise traverse a route that would cause the scann angle of its phased array antenna to drop below the predetermined lower limit.
It is still another object of the present invention to provide an apparatus and method for controlling movement of a planar, electronically scanned, phased array antenna used on a moving platform, and which is disposed on a movable support, in a manner which limits the frequency of incremental movement of the support. In this manner, wear and tear on the mechanism used for rotating the support can be significantly reduced while maintaining the scan angle of the antenna beam within a predetermined limit.
The above and other objects are provided by a method and apparatus in accordance with preferred embodiments of the present invention. The present invention is directed broadly to placement of a planar, electronically scanned, phased array antenna on a rotatable support and controlling rotational movement of the support as needed to maintain an elevation angle of the beam of the antenna at or above a minimum predetermined elevation angle, and while limiting movement of the support in a manner to reduce wear and tear on the mechanism used to rotate the support.
In one preferred form, the method of the present invention involves determining a maximum scan angle for the beam of the antenna, which is not to be exceeded, in order to maintain a desired bandwidth of the antenna at a given wavelength. The scan angle of the antenna beam is monitored during movement of the moving platform and, if the monitored scan angle exceeds the predetermined maximum scan angle, then the support is rotated in a direction necessary to point the antenna relative to the target being tracked (i.e., typically a satellite-based RF transponder) to cause the actual scan angle to be reduced below the maximum scan angle. The support for supporting the antenna is divided into a plurality of rotational positions, and preferably a plurality of evenly spaced apart rotational positions. Each rotational position is separated by a degree range which is determined such that movement of the rotational support from a given position to its next rotationally adjacent position causes the scan angle of the antenna to be reduced to a point below the maximum scan angle. In one preferred embodiment, the spaced apart rotational positions are spaced apart by no more than about 24xc2x0. The number of distinct rotational positions is also preferably limited to a relatively small plurality of positions, and in one preferred embodiment no more than about 20 such rotational positions, to which the support can be rotated.
In one preferred form of the present invention in which a dual element, electronically scanned phased array antenna is incorporated, the maximum scanned angle is determined by first determining the true-time-delay (TTD) increment between the elements of the antenna. This TTD increment is then used to determine the maximum scan angle. The direction of rotation of the support (i.e., either clockwise or counterclockwise) is determined by the sign of the azimuth angle of the antenna.
The method of the present invention accomplishes repositioning of the antenna periodically as needed to maintain the scan angle of the antenna below the maximum predetermined scan angle, and further with a minimum number of incremental position changes of the support. This significantly reduces wear and tear on the mechanism used to rotate the support and enhances reliability of such a system.